In recent years, by virtue of development of digital signal processing technology, it is possible to perform high-efficiency coding on high-volume digital information such as moving image, still image or audio information, recording of the data onto a small magnetic medium or small optical medium, and transmission of the data via communication medium. Image sensing devices or the like to easily obtain a high quality video image and immediately output the image to information medium by utilizing the above technology have been developed.
Especially, the MPEG coding technique is used in recent moving image coding. Since a code rate can be greatly reduced by using intra-frame coding for coding utilizing intra-frame correlation and inter-frame coding for coding utilizing correlation between preceding and subsequent frames, the method is widely used in video reproduction apparatuses as represented by a DVD and image sensing apparatuses as represented by a video camera.
In Japanese and U.S. television standards, the frame rate is about 30 frames per 1 second. However, the frame rate of film material video images used as movies is generally about 24 frames per 1 second. Accordingly, as shown in a reference document “ARIB STD-B20”, a video image, with a frame rate of about 24 frames per 1 second, is 2-3 pulled down to a frame rate of about 30 frames per 1 second. Then, the compatibility between film material video image and television video image is improved and MPEG coding is performed by a technique disclosed in Japanese Patent Application Laid-Open No. 2000-41244.
FIG. 6 is a block diagram showing an apparatus as a combination of the above conventional art to perform 2-3 pulldown and MPEG coding on a film material video image. In FIG. 6, reference numeral 801 denotes a video signal input terminal; 802, a coding unit; 803, a recording unit; and 804, a recording medium. The coding unit 802 has a 2-3 pulldown unit 810 and an MPEG unit 811.
A video signal having a frame rate of about 24 frames per 1 second, inputted from the video signal input terminal 801, is supplied to the coding unit 802. In the coding unit 802, the video signal is rate-converted by the 2-3 pulldown unit 810 to a rate of about 30 frames per 1 second (60 fields). The conversion is so-called 2-3 pulldown widely used in conversion from film materials for movies to television video images.
FIG. 7 is an explanatory view of the processing by the 2-3 pulldown unit 810. Numerals 901 to 910 denote a field stream of video image having a frame rate of about 24 frames per 1 second. Numerals 911 to 922 denote a field stream, converted from the field stream 901 to 910, having a frame rate of about 30 frames per 1 second. In the processing, the input video image is converted by 2 fields, to output video image having alternately repeated 2 fields and 3 fields. In the figure, the fields 901 and 902 are converted to the fields 911 and 912, then the fields 903 and 904 are converted to the fields 913, 914 and 915. In this repetition of processing, output 5 fields are generated by input 4 fields, thus conversion from 24 frames per second to 30 frames per second is realized. At this time, the first and third fields upon conversion from 2 to 3 fields are the same data. For example, the fields 913 and 915 are based on the field 903. Similarly, the fields 918 and 920 are the same data.
The rate-converted video signal is supplied from the 2-3 pulldown unit 810 to the MPEG unit 811. As recommended in the ISO/IEC 13818-2, the MPEG unit 811 compresses the video signal and generates a stream by a high-efficiency coding processing, and supplies the stream to the recording unit 803. The recording unit 803 records the generated stream on the recording medium 804.
As shown in FIG. 7, the MPEG unit 811 performs compressing based on parameters supplied from the outside. To remove redundancy of the 2-3 pulldown video image, parameters “top_field_first” and “repeat_first_field” are used. If the parameter “repeat_first_field” is “0”, it indicates a 2-field structure, while if the parameter “repeat_first_field” is “1”, it indicates a 3-field structure. As described above, since the first and third fields are the same, image data is not generated regarding the third field. On the other hand, the parameter “top_field_first” indicates whether the top or bottom field is the first field. If the parameter “top_field_first” is “0”, it indicates that the bottom field is the first field, while if the parameter “top_field_first” is “1”, it indicates that the top field is the first field.
FIG. 7 shows the relation between the field stream generated by the above-described pulldown processing and the parameters “top_field_first” and “repeat_first_field”. As it is understood from the figure, there are 4 combinations of these binary parameters and they are periodically repeated.
At normal recording times, coding is performed while the parameters are changed not so as to break the periodical repetition.
Next, a case where a new stream is connected to a stream already stored in the storage medium (data after the 2-3 pulldown and coding) and is recorded will be studied.
In this case, as the parameters of the already recorded stream and those of the newly connected stream are determined, a phase mismatch occurs in the parameters in a junction position. For example, in FIG. 7, the 2-3 pulldown stream has a 2-field end structure (end fields 921 and 922), and the 2 bit parameters “top_field_first” and “repeat_first_field” are (1,0). It is desirable that the parameters of the head picture of the new stream to be connected are (1,1), however, the values may not be “1” and “1”, rather, a phase mismatch occurs with ¾ probability.
To solve the above problem, the already-recorded stream or the new stream is re-encoded and the parameters are re-set so as to have a continuous phase. However, re-encoding takes processing time, and further, the image quality is seriously degraded by the re-encoding.